LT3041 datasheet(29/36 Pages) AD

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Data Sheet

LT3041

APPLICATIONS INFORMATION

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Rev. 0 | 29 of 36

Figure 86. Input and Output Noise Spectral Density of the LT3041

THERMAL CONSIDERATIONS

The LT3041 has internal power and thermal limiting circuits that

protect the device under overload conditions. The thermal shut-

down temperature is nominally 169°C with about 5°C of hysteresis.

For continuous normal load conditions, do not exceed the maximum

junction temperature, 125°C. It is important to consider all sources

of thermal resistance from junction to ambient, which includes

junction to case, case to heatsink interface, heatsink resistance,

or circuit board to ambient as the application dictates. Additionally,

consider all heat sources close to the LT3041.

The underside of the DFN package has exposed metal from the

lead frame to the die attachment. This package allows heat to

directly transfer from the die junction to the PCB metal to limit

maximum operating junction temperature. The dual, inline pin ar-

rangement allows metal to extend beyond the ends of the package

on the topside (component side) of the PCB.

For surface-mount devices, heat sinking is accomplished by using

the heat spreading capabilities of the PCB and its copper traces.

Copper board stiffeners and plated throughholes can also be used

to spread the heat generated by the LDO regulator.

Table 6 lists the thermal resistance as a function of the copper

area on a fixed board size. All measurements were taken in still

air on a 4 layer FR4 board with 1 oz solid internal planes and

2 oz top and bottom planes with a total board thickness of 1.6

mm. The four layers were electrically isolated with no thermal vias

present. PCB layers, copper weight, board layout, and thermal

vias affect the resultant thermal resistance. For more information

on thermal resistance and high thermal conductivity test boards,

refer to JEDEC standard JESD-51, JESD51-7, and JESD51-12.

Achieving low thermal resistance necessitates careful PCB layout.

Table 6. Measured Thermal Resistance for DFN Package

Copper Area

Board Area

Thermal

Resistance

Top Side1

Bottom Side

2500 mm2

34°C/W

1000 mm2

2500 mm2

34°C/W

225 mm2

2500 mm2

36°C/W

100 mm2

2500 mm2

37°C/W

1 The device is mounted on the topside.

CALCULATING JUNCTION TEMPERATURE

For example, given an output voltage of 3.3 V, an input voltage

of 5 V ± 5%, an output current range from 1 mA to 1 A, and

a maximum ambient temperature of 50°C, what is the maximum

junction temperature?

The power dissipation of the LT3041 is the following:

IOUTMAX× VINMAX−VOUT +IGND

×VINMAX

(7)

where:

IOUT(MAX) = 1 A.

VIN(MAX) = 5.25 V.

IGND (at IOUT = 1 A and VIN = 5.25 V) = 27 mA.

Therefore, PDISS = 1 A × (5.25 V − 3.3 V) + 27 mA × 5.25 V = 2.1

Using a DFN package, the thermal resistance is in the range of

34°C/W to 37°C/W depending on the copper area. Therefore, the

junction temperature rise above ambient approximately equals 2.1

W × 35°C/W = 73.5°C.

The maximum junction temperature equals the maximum ambient

temperature plus the maximum junction temperature rise above

ambient, which calculates as follows:

TJMAX=50°C+73.5°C=123.5°C

(8)

OVERLOAD RECOVERY

Like many IC power regulators, the LT3041 incorporates SOA pro-

tection. The SOA protection activates at input-to-output differential

voltages greater than 11 V. The SOA protection decreases the

current limit because the input-to-output differential increases and

keeps the power transistor inside a safe operating region for all

values of input-to-output voltages up to the Absolute Maximum

Ratings of the LT3041. The LT3041 provides some level of output

current for all values of input-to-output differentials. Refer to the

Figure 32. When power is first applied and input voltage rises, the

output follows the input and keeps the input-to-output differential

low to allow the LDO regulator to supply the large output current

and startup into high-current loads.

Due to current-limit foldback, however, at high-input voltages, a

problem can occur if the output voltage is low, and the load current

is high. Such situations occur after the removal of a short-circuit

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